Gears and gearing

Productivity of a system is characterized by uptime, output, and efficiency. Designers, component makers, and end users all contribute to (or hinder) a system's productivity. In this report, the editors of Motion System Design polled gearing experts for their advice on optimizing productivity with this particular component. Here are the responses on both sides of the issue, which we believe you'll find most helpful.

What particular design/construction features in gears and gear drives can REDUCE productivity?

Brian/Quality Transmission: This question might as well ask why a designer might select stock gears while blindfolded. Basically, the design and construction features that decrease productivity are simply those that aren't appropriate. For this reason, all gear systems should be thoroughly analyzed to determine the proper sizing for the gears, because analysis determines the materials and geometry most suitable for the design.

Chris/DieQua: With respect to gear drives, productivity can be measured as anything from the cost of operation to the actual output performance, depending on the goal to be achieved. Design elements to be considered are gear quality, bearing quality, housing, bearing support, and shaft machining tolerances, seal design, lubrication, and mounting. Poor-quality components and assembly techniques can affect productivity by reducing efficiency, rotary motion accuracy, shock load absorption, and operational life.

Georg/Intech: Just like metal gears, plastic gears can be sized for a given set of parameters — torque, rpm, temperature, shock load, and expected life. The original stress and strain calculations for these plastic gears were based on those modeled for traditional tooth root stress on lubricated metal gears. However, with plastic gears there is another parameter that can affect productivity: flank wear. In fact, this type of wear is an important factor in all non-lubricated gears — not just those that are plastic.

Advancements in tooth profile design (in other words, tooth modifications) have steadily reduced friction and wear in the mesh, which has opened the way for the use of plastic gears in high load, high speed applications, typically reserved for metal gears.

Anthony/Harmonic Drive Technologies: Many machines run with reciprocating motion, requiring high duty cycles and reversing loads. If the gears specified for such a system can't handle the great amount of stress this produces, it can cause wear, failure, and decreased productivity over a short period of time.

What particular design/construction features in gears and gear drives can INCREASE productivity?

Georg/Intech: Let's talk about open gears for a minute — those gears not enclosed in a box, but rather, exposed to the outside environment. In these situations, lubrication can get on products, and can contaminate the other way as well — by attracting dirt to gears. For this reason, self-lubricating gears are increasingly finding their place in these open drives — in packaging, paper processing, printing, and medical applications — both as replacements for existing metal gears and in new OEM designs.

Alan/Harmonic Drive Technologies: Less backlash in gears means more motion control and accuracy. The unique tooth mesh of harmonic drives (with teeth in constant contact at the major axis) minimizes the effects of frequent starts, stops, and reversals, allowing the gears to last for several years without failures. In short, their zero-backlash characteristic allows control engineers to run their machines at increased cycle rates, reducing down time and increasing productivity.

What can designers do to INCREASE or inadvertently REDUCE the productivity of the gears and gear drives they place in machines?

Anthony/Harmonic Drive Technologies: Design engineers have to do their homework and must understand what they are trying to achieve with their gear systems. They must consider all variables and scenarios when choosing a gear system. The possibility of collisions (those unexpected torque peaks from emergency stops and crashes) must be considered. If this possibility isn't considered, it can instantly damage mechanical components. Most importantly, designers should talk to applications engineers who can offer expert advice on which size or type of gear will allow a machine to operate at peak performance.

Chris/DieQua: The two main things designers do to inadvertently reduce gear drive productivity is inappropriate drive selection and improper gear drive mounting. Always keep in mind there are many kinds of gear drives, each with their own advantages and disadvantages; selecting the correct drive is critical. Further, within each gear type there are levels of quality. Choosing the correct quality level is important in achieving maximum productivity, even when the proper gear drive is selected.

Brian/Quality Transmission: However, I'd like to point something out. If a gear design is poor, then it runs inefficiently — but there is no direct correlation that this will necessarily decrease the “productivity” of the gears. In other words, a gear train that needs to reduce a speed from 100 rpm to 10 rpm can do so efficiently or inefficiently, but as long as the 10:1 reduction occurs, its productivity on a micro-scale remains intact. For this reason, on the gear-component level, this is really a question of efficiency.

Georg/Intech: Design engineers whose machine specs call for self-lubricating gears should talk to an application engineer early in the design stage. If sized correctly, plastic gears can carry the torque of metal gears in most applications, offering significant productivity improvements with the added benefit of low maintenance and contamination-free product output.

How do end users inadvertently DECREASE productivity from the gears and gear drives on their machines?

Chris/DieQua: Many users improperly mount gear units. In fact, lack of precise shaft alignment, excess radial loads, and not considering bearing lubrication in vertical shaft orientations all lead to reductions in operational longevity.

Brian/Quality Transmission: How end users reduce productivity from their gear system goes back to the initial analysis of what the system is being asked to do. If the initial assumptions have changed, then the gears will need to change. If the end user is not lubricating the gears properly, if the shaft alignment is not maintained, if the shock loading is unaccounted for, if the speeds are increased, or if life span was not pre-determined for periodic maintenance, productivity will be halted while the gears are being replaced.

What can end users do to INCREASE productivity from the gears and gear drives on their machines?

Alan/Harmonic Drive Technologies: End users have to understand the full requirements of the gears in their machines. To achieve maximum performance, it is important that they understand the full operating ranges, maximum capabilities, proper maintenance schedules, and the effects of environmental conditions on the gears.

Georg/Intech: The metal cores on plastic gears can increase productivity. They allow for a secure, metal-to-metal gear attachment to the shaft using a keyway, QD bushing, set screw, press fit or other means common for metal gears. They allow a higher torque transmission and in high-speed applications, they dissipate heat and reduce thermal expansion of the plastic gear portion. In corrosive environments or where frequent wash downs are required stainless steel core is available. To reduce inertia — in other words, in stop-and-go applications or on speedy machines, designers specifying plastic gears would do best to specify aluminum cores. Metal cores in general allow a reduced backlash in the tooth mesh.

Chris/DieQua: Even when an appropriate gear drive is applied properly, the end user can reduce a system's productivity. Not adhering to recommended maintenance cycles, increasing pulley belt tensions, exceeding machine speed parameters, and not keeping the drive clean (hindering heat dissipation) all lead to reductions in gear drive productivity. Also, when replacing a drive on an existing machine, end users rarely realign the new unit. Because mounting tolerances vary from unit to unit, exceeding alignment tolerances often results in premature bearing wear.

Anthony/Harmonic Drive Technologies: We often recommend that lubricant be considered first. Once a lubricant is chosen, sizes and materials can be determined to best suit requirements. Harmonic drives can be configured to maintain their performance under wide operating conditions and environmental extremes. For example, harmonic drives have been configured to operate in:

Drilling rigs with temperatures up to 350° and pressures to 20,000 psi

Semiconductor clean room environments (up to 100 count)

Deep space aerospace extremes (temperature and vacuum)

Georg/Intech: Plastic gears don't require lubrication — a form of maintenance — and that translates to increased productivity. Plastic gears also reduce noise by 6 dB, which can make for a more efficient human environment.

Give one example (with respect to gears and gear drives) of how a designer or end user reduced machine productivity by poor design/maintenance practices.

Chris/DieQua: In a printing press application, the designer was looking to reduce the cost of the driveline. In this case a series of bevel reducers drove the print cylinders. The lower cost gear drives selected did not provide the proper transmission error and rotary motion accuracy that was necessary. The motion variations were transmitted directly to the print cylinders negatively affecting print registration. While the proper gear type was selected, the appropriate quality level was not. The losses in productivity far outweighed the driveline cost savings.

Alan/Harmonic Drive Technologies: All designers today are trying to minimize space and reduce costs. For this reason, there is usually a space constraint when it comes time to design in a gear drive. Applications engineers, as a result, are forced to scramble and find a gear drive that meets customer requirements and fits in the provided space. This is not always an easy fix and sometimes requires a custom unit.

Give one example (with respect to gears and gear drives) of how a designer or end user increased machine productivity with good design/maintenance practices.

Alan/Harmonic Drive Technologies: The old saying you get what you pay for definitely applies to gear drives. Many times designers need a compact zero-backlash, high-torque gear system. But they balk at the price because of budgetary constraints and end up sacrificing the performance they require. This usually causes additional problems when a machine does not function properly. Most designers in this situation end up calling back and buying the gear originally recommended. The bottom line is not to let price be the single motivating factor when designing in a gear system. Design engineers must know what their requirements are and spec in a unit that will provide the necessary performance and limit downtime over the long term.

Anthony/Harmonic Drive Technologies: Design engineers have to talk to applications engineers throughout the design process. Their experience with special materials, lubrication, and custom designs can expedite the design process and alert designers to potential problems and pitfalls. Applications engineers are also aware of the choices available to design engineers and can offer alternate suggestions that might improve the design. For example, an applications engineer could offer a complete gearbox, actuator, or a simplified mounting interface in place of a standard gear component, saving the designer the time and effort of having to design his own housing.

Georg/Intech: I'll give a very specific example. On the main drive gear of a diaper-making machine, a lubrication-free gear was required because product contamination was unacceptable. In the end, a plastic gear with an aluminum core was chosen to dissipate the heat generated from the high pitch-line velocity application. An added requirement specified that the gear has to withstand a jam or a sudden stop, (basically, an extreme-shock load) and still have a safety margin. For this, the gear was designed with a tooth profile modification to reduce flank friction.